The rear flex disk is very beefy and robust compared to the front flex disk, which is almost always the one that is bad. The minor cracks in the rear disk you report are probably not a problem.

Can't make any sense out of your driveshaft slack difference between the front and rear, but the way to check them is to get the car up in the air on four jackstands under the four jack pads so the suspension is hanging free and tires are off the ground. To check the front flex disk, place the trans in Park (or first or reverse on a five speed), parking brake OFF and rotate the driveshaft with your hand. The front disk is much softer and flexible than the rear and, if bad, will show air gaps between the bolt bushings and rubber as you torque the driveshaft back and forth. It's tough to see the front flex disk because it's in front of the vibration damper.

To check the rear, place the trans in neutral, firmly pull on the parking brake and torque the driveshaft with your hand looking for play.

I consider front flex disks to be automatically suspect if they have more than 60K miles or normal driving on them. They're basically a 60K mile service replacement item!

Duke